1. Field of the Invention
The present invention generally relates to a color cathode ray tube and, more particularly, to a mounting of a perforated shadow mask in the color cathode ray tube.
2. Description of the Prior Art
It is well known that the color cathode ray tube utilized as a display of, for example, a television receiver set employs a generally rectangular perforated shadow mask assembly including a perforated shadow mask which is made of a perforated thin metal plate or foil. The perforated thin metal plate or foil, that is, the perforated shadow mask, has a multiple of minute circular or elongated apertures defined therein in a predetermined pattern corresponding to the triads of primary color elemental phosphor dots on the inner surface of the faceplate which forms a part of the highly evacuated envelope.
When it comes to the manner by which the rectangular perforated shadow mask is supported inside the faceplate in the vicinity of the luminescent phosphordeposited screen, a generally rectangular rigid support frame is employed so as to intervene between the periphery of the perforated shadow mask and the faceplate.
More specifically, as shown in FIG. 1 of the accompanying drawings showing the conventional color cathode ray tube in exploded view, the color cathode ray tube comprises a highly evacuated envelope including a generally rectangular faceplate 1, a funnel section 3 and a generally cylindrical neck section 4, and an electron gun assembly 6 housed within the neck section 4 for emitting three electron beams of three primary elemental colors, that is, red, green and blue. The funnel section 3 is closed at one end by the neck section 4 and at the other end by the faceplate 1. The faceplate 1 is generally in the form of a cup-shaped envelope cap including a generally spherical screen plate 1a and a peripheral side wall assembly 1b which protrudes generally perpendicular to the screen plate 1a in a direction generally parallel to the longitudinal axis of the envelope shown by the arrow Z and which is sealed to the funnel section. The inner surface of the screen plate 1a confronting the interior of the evacuated envelope has a substantially rectangular area, that is, a substantially rectangular phosphor deposited area 1c (FIG. 3), where triads of phosphor dots are deposited in a predetermined pattern.
A perforated shadow mask assembly generally identified by 2 in FIG. 1 is positioned inside the side wall assembly 1b of the faceplate 1 and retained in position by means of a plurality of mounting spring members 5, the details of said perforated shadow mask assembly 2 being shown in FIG. 2. As best shown in FIG. 2, the perforated shadow mask assembly 2 comprises a generally rectangular color selection electrode member or finely perforated shadow mask 21 having a perforated plate and a mounting skirt 23 generally perpendicular to the perforated plate, and a support frame 22, said mounting spring members 5 being fixed exteriorly to the support frame 22. The perforated shadow mask 21 is retained in position inwardly of the support frame 22 with the mounting skirt 23 received in and welded to the support frame 22 while confronting, and spaced a predetermined distance inwardly from, the screen plate 1a.
As is well known to those skilled in the art, the finely perforated shadow mask 21 has the perforated plate which is of a generally spherical shape similar to the shape of the screen plate 1a of the faceplate 1. In order for the perforated shadow mask 21 to be supported rigidly by the support frame 22, the support frame 22 generally used in the conventional cathode ray tube is made with the use of either hot rolled thin steel plate or cold rolled thin steel plate having a thickness within the range of 0.8 to 2.5 mm, into a generally rectangular shape as shown in FIG. 2.
The perforated shadow mask 21 is operable to allow the selective passage therethrough of the electron beams emitted from the electron gun assembly 6 and, for this purpose, the perforated plate of the shadow mask 21 is formed with a multiple of minute circular or elongated apertures 24, for example, either circular through-holes or slots, defined therein in a predetermined pattern corresponding to the triads of primary color elemental phosphor dots on the inner surface of the screen plate 1a. That area of the perforated plate of the shadow mask 21 where the minute circular or elongated apertures 24 are actually formed is substantially rectangular in shape as shown by the phantom line 25, which area is hereinafter referred to as a perforated area.
As far as the manufacture of the color cathode ray tube is concerned, the minute apertures 24 defined in the perforated plate of the shadow mask 21, which operate to allow the selective passage of the electron beams during the operation of the color cathode ray tube, are utilized as a registration member for registering the substantially rectangular phosphor deposited area 1c of the screen plate 1a during an exposure step during which exposure lights are allowed to pass through the minute apertures 24 for the purpose of formation of the substantially rectangular phosphor deposited area 1c on the screen plate 1a in a centered fashion with respect to the substantially rectangular apertured area 25 of the shadow mask 21.
If, in the course of manufacture of the conventional color cathode ray tube of the above described construction, the perforated shadow mask 21 is supported with the rectangular perforated area 25 thereof laterally displaced or rotated relative to the screen plate 1a of the faceplate 1, at the time of execution of the exposure step in which the substantially rectangular phosphor deposited area of the screen plate 1a is exposed to the exposure lights, the resultant rectangular phosphor deposited area 1c of the screen plate 1a would have its center P displaced laterally in a plane generally perpendicular to the longitudinal axis Z of the evacuated envelope (that is, in an X-Y plane containing X- and Y-axes perpendicular to each other and also perpendicular to the longitudinal axis Z of the evacuated envelope) from the position Po (or the center of the screen plate 1a) where it ought to have occupied as shown in FIG. 3(a), or would be angularly offset relative to the screen plate 1a about the longitudinal axis Z of the evacuated envelope as shown in FIG. 3(b). Once the rectangular phosphor deposited area 1c is formed on the inner surface of the screen plate 1a in such a distorted fashion, the resultant color cathode ray tube would exhibit an undesirable displaced reproduction of televised color pictures, rendering the color cathode ray tube to be unacceptable for commercial use.
One of the major causes of the displaced reproduction of the televised pictures will now be discussed. In the case of the color cathode ray tube wherein the perforated shadow mask 21 is fixedly welded at its peripheral edge to an inner wall face of the support frame 22 through the skirt 23 such as discussed hereinbefore, it may often occur that, because of the presence of an insufficient and/or uneven clearance between the the the skirt 23 integral with the perforated shadow mask 21 and the inner wall face of the support frame 22 to which it is welded, the perforated shadow mask 21 may be fixed in position inside the support frame 22 in a manner laterally displaced or angularly offset relative to the support frame 22 as shown in FIG. 3(c) or FIG. 3(d).
By way of example, in the case of the 37-inch color cathode ray tube, the rectangular aperture area 25 of the perforated shadow mask 21 generally has a width of about 640 mm as measured along any one of the opposite long sides thereof. If the perforated shadow mask 21 is fixed to the support frame 22 while having been angularly offset, or rotated, about the longitudinal axis Z of the evacuated envelope and inclined at an angle of 0.4.degree. at relative to the X-axis passing through the center Po of the screen plate 1a, the amount of angular offset of the rectangular phosphor deposited area 1c on the screen plate 1a as indicated by d in FIG. 3(b) would be about 5 mm, resulting in the undesirable rotated reproduction of color pictures.
In order to avoid the distorted reproduction of the televised pictures, the perforated shadow mask has to be accurately and precisely supported in position inside the support frame without being laterally displaced or angularly offset. This is particularly true where the color cathode ray tube employs the perforated shadow mask assembly of a type wherein the perforated shadow mask is welded to the support frame.
According to the prior art, the use is required of complicated and expensive centering and/or positioning machines and equipment for accurately and precisely positioning the perforated shadow mask relative to the support frame during the assembly of the perforated shadow mask assembly. Considering the presence of deviations from a batch of cathode ray tubes to another during the manufacture thereof, the use of the complicated and expensive centering and/or positioning machines and equipment does not pay.
The U.S. Pat. No. 4,327,307, issued Apr. 27, 1982, to Penird et al., discloses a perforated shadow mask assembly for color cathode ray tubes which comprises a plurality of spaced protuberances formed on the support frame for providing the welding locations for the perforated shadow mask and thus providing a space between the wall of the support frame and the skirt of the perforated shadow mask so that both of the stresses normally introduced into the perforated shadow mask when it is welded to the support frame and the tendency of the perforated shadow mask to dome during heating can be reduced. The perforated shadow mask disclosed therein has the skirt telescoped over and welded to the protuberances formed on the support frame.
In any event, the purpose of use of the protuberances in said U.S. Patent is, as clearly discussed therein, to relieve the strain normally applied to the perforated shadow mask during the welding and also to reduce the effect of doming. As is well known to those skilled in the art, the doming of the perforated shadow mask is the phenomenon in which the perforated shadow mask expands outwardly towards the phosphor deposited screen under the influence of heats evolved in the perforated shadow mask assembly as a result of impingement of electron beams during the passage thereof through the apertures in the perforated shadow mask. Accordingly, it is clear that the invention of the U.S. Patent is not useful for repositioning of the perforated shadow mask accurately and precisely relative to the phosphor deposited area on the screen plate.